In waveguide resonators, the resonator walls influence the propagation of a laser beam and shape its mode, to a certain extent. A waveguide resonator is defined by the so-called Fresnel number, NF=a2/(λL)<0.5, where a is half of the resonator aperture, λ the wavelength of the laser beam, and L is the resonator length. One significant advantage of using a waveguide resonator to form a laser beam lies in the fact that it allows a decrease in the transverse dimensions of the laser beam inside the waveguide. Small transverse dimensions increase the diffusive cooling efficiency of a laser gas bounded by the waveguide. A narrow bore also allows efficient transportation of waste heat to the side walls of the resonator. With such efficient cooling, the plasma temperature is lower, increasing gain, and the pressure of the laser gas may be increased. This leads to higher power per gas volume and faster optical response to RF pulses of pump energy.
Area-scaled “slab” waveguide lasers take advantage of efficient cooling in the waveguide axis, while allowing the other axis to behave with “free-space” characteristics. In order to couple power out of the cavity, the resonator may be designed as an “unstable” resonator in the free-space direction. Such a resonator may take the form of a negative or positive branch unstable resonator. For stability reasons, the negative branch is more popular. Light is coupled out by allowing recirculating energy bouncing between the mirrors to “walk off” the edge of one mirror. The out-coupling percentage may be controlled by the ratio of the different curvatures of the front and rear mirrors in the free-space direction. While diffusion cooled slab waveguide gas lasers deliver high output power with relatively small size and low complexity, such lasers traditionally produce laser beams having asymmetrical transverse profiles (e.g., elliptical cross-sectional profiles) that possess uneven and relatively high divergence in one transverse axis compared to the other. In conventional slab waveguide lasers, these undesirable beam properties have commonly been corrected by incorporating beam shaping optics, including at least one cylindrical lens. Additionally, a spatial mode filter has often been used to eliminate satellites on the beam edges in the free-space unstable-resonator direction. These additional optical elements typically increase production costs and decrease output power through absorption.